On expressways and other multi-lane roads, for vertical traffic signs such as direction signs and traffic lights to be clearly visible to vehicles on the outside (fast) lanes they cannot be erected only at the edge of the expressway, but must be mounted on overhead gantries which extend over the roadway. Such overhead gantries are also known as trestle bridges or overhead information signs.
No really satisfactory solution exists as yet where temporary expressway signs are required. When permanently installed traffic signs have to be altered temporarily a provisional sign or a fabric covering is often hung over the traffic sign with the aid of a vehicle equipped with a lifting platform.
Apart from the disruption to traffic involved in this procedure, the rapid or temporary erection of traffic signs over the roadway is not solved where there is no permanently installed overhead gantry. This situation can, for example, arise when sudden diversions, unusual destinations or the like have to be signposted.